Regeneration of the purification beds used in olefin polymerization processes requires a high flow of nitrogen or other heat transfer gas, but for a limited number of hours per year. The trend in the art is to use once-through nitrogen as an inert heating and cooling fluid that also delivers any regeneration gas and carries away the evolved impurities to the flare. The high absolute and peak nitrogen use and flaring increases operating and investment costs.
An alternative design was developed that utilized a closed nitrogen recirculation cycle. Unfortunately, the investment cost was very high due to the need to cool the recirculating gas before compression in a blower or mechanical compressor. This cooling also can cause the condensation of water that is a by-product of the dryer regeneration. This requires additional investment for separation and disposal. There is also a concern that the recirculated residual hydrocarbons may “coke out” on the very high temperature heating elements of an in-line electric heater. Since impurities are evolved, the cycle cannot be completely closed during some steps of the regeneration.
U.S. Pat. No. 4,424,144 (Pryor et al.) described shaped products of a 3A zeolite formed either as beads or extrudates without any binder remaining. Both the beads and the extrudates can be used to dry a mixture of a hydrocarbon compound such as ethylene and water. The patent describes how the zeolite may be regenerated by purging with nitrogen in a laboratory bomb. Published U.S. Pat. Applications 20040106752 and 20050043497 (Gindelberger et al.) describe a molecular sieve purification bed used to purify ethylene on a laboratory scale prior to polymerization. However, there appears to be no discussion of using inert gas at pipeline pressure, or open-loop methods or systems for regenerating olefin purification beds, in these patent publications.
Thus, there is a need in the art to reduce the above-mentioned burdens of once-through nitrogen regeneration methods, and avoid the cost and problems associated with closed-loop methods of regenerating purification beds.